Junction box thermal management

ABSTRACT

An electrical assembly may include an enclosure having a base portion to attach the enclosure to a panel and a heat dissipating portion opposite the base portion, a circuit board having a first thermal interface on a first side of the board, a second thermal interface on a second side of the board, and a thermally conductive portion to provide enhanced thermal conduction between the first thermal interface and the second thermal interface, a power electronic device having a thermal interface coupled to the first thermal interface of the circuit board, a heat spreader arranged to transfer heat to the heat dissipating portion of the enclosure, and a thermally conductive pad coupled between the second thermal interface of the circuit board and the heat spreader.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional Patent Application Ser. No. 61/330,840 titled Electrical Box Thermal Management filed May 3, 2010, and U.S. Provisional Patent Application Ser. No. 61/345,984 titled Serviceable Electrical Box Thermal Management filed May 18, 2010.

BACKGROUND

FIG. 1 is a simplified cross-sectional view of a prior art junction box for a solar panel that utilizes photovoltaic (PV) cells to convert solar energy into electric power. The junction box 10 includes a plastic box 12 having a base portion 14 for mounting the box to a solar panel 16. A plastic cover 18 engages the box to seal off the inside of the box from moisture, dust and other contaminants. A printed circuit board 20 includes circuitry for making connections between conductive leads from the panel and cables that carry power to a power distribution system. The conductive leads and cables are omitted from FIG. 1 to avoid obscuring the remaining structure of the junction box.

The printed circuit board 20 also includes one or more bypass diodes 22 which are used to prevent damage to the solar panel or other components and improve efficiency under certain operating conditions. The diode 22 shown in FIG. 1 has a plastic package body 24 and a metallic slug 26 which acts as a heat sink to dissipate heat generated within the diode body. The diode 22 is electrically connected to the printed circuit board 20 through wire leads 28.

To prevent damage to the diode if it overheats, the plastic body of the diode is positioned in physical contact with a thermally conductive plate 30 which is bonded to the cover 18. The thermally conductive plate 30 is fabricated from a metal such as copper or aluminum and acts as a heat spreader to distribute heat from the diode across a relatively large surface area of the cover, thereby enabling heat from the diode to be dissipated through the cover of the junction box.

The diode is designed to dissipate heat primarily through the metallic slug which forms a thermal interface with the printed circuit board. Since the printed circuit board only has a limited capacity to dissipate heat, the conductive plate enables some additional heat to be dissipated through the plastic body of the diode, which has relatively low thermal conductivity. Thus, the prior art arrangement of FIG. 1 fails to fully utilize the primary heat dissipating mechanism of the diode, and it provides poor thermal management.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art junction box for a solar panel.

FIG. 2 illustrates an embodiment of an electrical assembly according to some inventive principles of this patent disclosure.

FIG. 3 illustrates another embodiment of an electrical assembly according to some inventive principles of this patent disclosure.

FIG. 4 illustrates an example embodiment of an electrical assembly according to some inventive principles of this patent disclosure.

FIG. 5 illustrates another example embodiment of an electrical assembly according to some inventive principles of this patent disclosure.

FIG. 6 illustrates an embodiment of a cover for an electrical enclosure according to some inventive principles of this patent disclosure.

FIG. 7 illustrates an embodiment of an electrical assembly including planar magnetics according to some inventive principles of this patent disclosure.

FIG. 8 illustrates an embodiment of an electrical assembly including through-board mounted components according to some inventive principles of this patent disclosure.

DETAILED DESCRIPTION

FIG. 2 is a cross-sectional view of an embodiment of an electrical assembly according to some inventive principles of this patent disclosure. The embodiment of FIG. 2 may be suitable for use as a junction box for a solar panel, but the inventive principles are not limited to solar panels.

Referring to FIG. 2, the electrical assembly includes an enclosure 32 having a base portion 34 to attach the enclosure to a panel and a heat dissipating portion 36 opposite the base portion. A circuit board 38 has a first thermal interface 40 on a first side of the board, a second thermal interface 42 on a second side of the board, and a thermally conductive portion 44 to provide enhanced thermal conduction between the first thermal interface and the second thermal interface. A power electronic device 46 has a thermal interface 50 coupled to the first thermal interface 40 of the circuit board 38. A heat spreader 52 is arranged to transfer heat to the heat dissipating portion 36 of the enclosure 32. A thermally conductive pad 54 is coupled between the second thermal interface 42 of the circuit board 38 and the heat spreader 52.

FIG. 3 is a cross-sectional view of another embodiment of an electrical assembly according to some inventive principles of this patent disclosure. The embodiment of FIG. 3 may also be suitable for use as a junction box for a solar panel, but the inventive principles are not limited to use with solar panels.

Referring to FIG. 3, the electrical assembly includes a box 56 having a base portion 58 to attach the enclosure to a panel and an opening opposite the base portion. The assembly also includes a cover 60 to engage the opening of the box. A circuit board 62 has a first side, a high-profile side 64, with at least one power electronic device 66 mounted thereon, and a second side, a low-profile side 68. A thermally conductive plate 70 is mounted adjacent to the second side 68 of the circuit board 62. The circuit board is constructed and arranged to transfer heat from the power electronic device to the thermally conductive plate as shown by arrows 72. The thermally conductive plate is arranged to transfer heat to the cover when the cover is engaged with the opening of the box as shown by arrows 74.

The high-profile side 64 of the circuit board may include additional components such as 67A and 67B having relatively high-profiles, while the low-profile side 66 may have components with a relatively small profile mounted thereon.

FIG. 4 illustrates an example embodiment of an electrical assembly showing some possible implementation details according to some inventive principles of this patent disclosure. The embodiment of FIG. 4 is described in the context of a junction box for a solar panel, but the inventive principles are not limited to use with solar panels or to any of the implementation details shown. The embodiment of FIG. 4 is not shown to scale, and some of the dimensions are exaggerated for purposes of illustration.

Referring to FIG. 4, the junction box includes a box 76 having sidewalls 78 and 80, and a base 82 for mounting the junction box to a solar panel 86. Connections from the solar panel are made through conductive leads (which in this example are ribbon conductors) 90 that pass through one or more openings 88 in the base. A cover 92 seals the junction box from moisture, dust and other contaminants, and also dissipates heat that is generated by components inside the box. The box 76 and cover 92 may be fabricated from plastic such as ABS or any other suitable nonconductive material. A conductive material may be used for the box and/or cover, but additional measures may be needed to provide electrical isolation from the other components.

A printed circuit board 94 includes a first metal layer 96 and a second metal layer 98 which are etched to form any suitable circuits on the board. The first and second layers 96 and 98 are shown in sections or traces 96 a, 96 b, etc., and 98 a, 98 b, etc., due to patterning that is exposed by the cross-sectional nature of the drawing. The first and second layers may also be referred to as low-profile and high-profile layers, respectively, in reference to the types of components that may be mounted on the board as explained below. The first and second layers may also be referred to, somewhat arbitrarily, as top and bottom layers, respectively. Likewise, the designations of first and second are also arbitrary. The board may also include any other number of conductive layers between the first and second layers.

The second or bottom side 100 of the board 94 is also referred to as the high-profile side of the board because it may be populated with relatively high-profile components such as microcontrollers, logic chips, gate arrays, transformers, large capacitors, large inductors, switching transistors, power diodes, transceiver chips, etc., that may be used to implement any suitable electric circuitry that may be housed within the enclosure of the junction box. The circuitry may be as simple as one bypass diode, or it may include one or more complete power converters such as a DC/DC power optimizer, one or more DC/AC panel-level, string-level, or cell-level microinverters, etc.

For purposes of illustration, the embodiment of FIG. 4 is assumed to include a DC/DC converter with local power optimization including maximum power point tracking (MPPT) for the solar panel 86. The converter circuitry includes a power inductor 104 having leads soldered to traces 98 a and 98 b. The converter circuitry includes other power and control components that are not visible in this cross-sectional view. A bypass diode 106 has a plastic body 108 and metallic slug 110 which forms a thermal interface. The bypass diode 106 is electrically connected to traces such as 98 c through leads such as 112.

A portion 98 d of the bottom metal layer forms a solder pad and thermal interface for the circuit board. Thus, with the diode 106 soldered to pad 98 d, the thermal interface 98 d for the bottom of the board is coupled to the thermal interface 110 of the diode to facilitate highly effective heat flow from the diode to the circuit board.

The solder pad 98 d is thermally coupled to another solder pad 96 d on the opposite side of the board through a thermally conductive portion of the board to provide enhanced thermal conduction between the bottom thermal interface 98 d and the top thermal interface 96 d. In the embodiment of FIG. 4, the thermally conductive portion of the board is implemented with thermal vias 114 which are plated-through holes that connect the two pads.

The first or top side 102 of the board 94 is also referred to as the low-profile side of the board because it may be populated with relatively low-profile components 116 and 118 such as resistors and small capacitors, signal diodes, small transistors, etc. For example, in some embodiments, the components on the low-profile side of the board may be limited to a height of about one millimeter (1 mm) or less.

Solder pads 96 e and 98 e provide a connection point for connecting the ribbon leads 90 to the circuit board 94. In this example, solder 120 is shown as the connection technique, but any other suitable technique such as spring clips, screws, etc. may be used.

A thermally conductive plate 122 is coupled to the top thermal interface 96 d through a thermally conductive pad 124. An example of material suitable for the pad 124 is Bergquist Gap Pad. The thermally conductive plate operates as a heat spreader to eliminate hot spots and is arranged to transfer heat to the cover 92 which dissipates heat from the junction box and away from the solar panel 86. The plate 122 may be attached to the circuit board 94 in any suitable manner. For example, if the plate is made of copper or aluminum, screws may pass through holes in the circuit board and be threaded into the plate with spacers used to maintain the distance between the board and the plate.

An electrically insulating sheet 126 may be included between the plate 122 and the circuit board 94 to prevent shorting of traces or components on the low-profile side of the board. In this view, the sheet 126 is shown as sections 126 a and 126 b due to the hole through which the thermally conductive pad 124 passes. An example of material suitable for the electrically insulating sheet is polycarbonate or polypropylene sheet such as Formex polypropylene sheet from ITW Formex.

The embodiment of FIG. 4 may also include power connections for connecting the DC/DC converter to a power distribution system. Such connections are not shown in FIG. 4 so as not to obscure the other inventive features, but the power connections may be made in any suitable manner such as through solder connections to the circuit board, screw terminals, etc.

The circuit board and thermally conductive plate may be mounted in and/or to the box 76 and or cover 92 in any suitable manner. For example, if the plate is attached to the circuit board as described above, the circuit board may be mounted to the box with standoffs and screws. Alternatively, the plate may be mounted to the box using tabs on the plate that slide or snap into slots and/or resilient structure. As another example, the plate may be mounted to the cover with screws, double-sided tape, adhesives, etc.

The inventive principles illustrated with respect to the embodiment of FIG. 4, as well as the other embodiments disclosed herein, may provide numerous benefits and advantages. For example, circuit boards are typically fabricated with all or most of the high-profile components on one side. The high-profile components may include components that dissipate significant heat such as power electronic devices like bypass diodes and switching transistors. It may be difficult, however, to provide individual heat sinking to these components in a manner that enables the heat to be removed from the interior of the junction box. The embodiment of FIG. 4, however, may provide a highly effective thermally conductive path from the bypass diode through the circuit board, the thermally conductive pad, the thermally conductive plate and out through the cover of the junction box. This may eliminate the need for individual heat sinks while also eliminating hot spots within the junction box. It may also enable the use of a single heat spreader to be used with multiple pads and heat dissipating components, thereby reducing assembly costs and improving reliability.

Although the inventive principles are not limited to use with solar panels, they may provide some features that are particularly beneficial when applied to solar panels. For example, if an electronic module embedded in a solar panel creates a hot spot on the PV array, it may increase the mismatch between PV cells, thereby degrading performance and reducing reliability due to higher localized temperature cycles. The inventive principles may enable most of the heat generated within a junction box module to be radiated away from the panel, thereby improving performance and long-term reliability. Moreover, the inventive principles may enable the implementation of enclosures having lower height profiles which may be critical due to mounting, regulatory and cost constraints.

FIG. 5 is an exploded view illustrating another example embodiment of a junction box according to some inventive principles of this patent disclosure.

The embodiment of FIG. 5 includes a plastic box 130 having a base portion 132 to attach the junction box to panel connectors 134 to secure cables 136 to the box for connecting the circuitry inside the junction box to a power distribution system. Screw terminals 138 enable the cables to be connected to solder-lug connectors on the printed circuit board 146. Hooks 140 engage tabs 141 on thermally conductive plate 144, while resilient latches 142 engage additional tabs 143 on the plate to enable the plate to be snapped into the box.

Channels 148 in the box enable ribbon leads from a solar panel to pass through the box to slots 150 on the circuit board 146 where they may be soldered to pads on the board. The printed circuit board 146 includes numerous low-profile components on the top side, and a few high-profile components such as transformer 152 and inductor 154 are visible on the high-profile side of the board. Seven thermally conductive pads 156 are attached to the board at locations having through vias and solder pads which provide thermally conductive connections to power electronic devices such as bypass diodes that are located on the high-profile side of the board but are not visible in the view of FIG. 5.

An electrically insulating sheet 158, which is positioned between the circuit board and plate includes openings 160 to enable the thermally conductive pads 156 attached to the circuit board to pass through the insulating sheet and contact the thermally conductive plate.

The circuit board is attached to the thermally conductive plate using standoffs and screws, while capturing the insulating sheet between them.

Plastic cover 162 has resilient tabs that engage corresponding channels 166 on the box to secure the cover on place on the box.

FIG. 6 is a cross-sectional view of an embodiment of a box and cover for an electrical enclosure according to some inventive principles of this patent disclosure. The embodiment of FIG. 6 includes a box 32 and a cover 170 having an o-ring 172 disposed in a groove that runs around the entire circumference of the cover. The groove is formed between two rims 174 and 176 which contain the o-ring and are spaced to receive the side walls 178 of the box when the cover is moved into position on the box as shown by arrow 177. The presence of a thermally conductive plate 180, such as those described above, may necessitate clearance or creepage distance to the outside of the box to meet safety agency requirements. Therefore, the inner rim 176 is elongated in a direction perpendicular to the cover to provide increased creepage distance as shown by arrow 182.

FIG. 7 is a cross-sectional view illustrating an embodiment of an electrical assembly including planar magnetics according to some inventive principles of this patent disclosure. The embodiment of FIG. 7 includes an enclosure 184 having a base portion 186 to attach the enclosure to a panel and a heat dissipating portion 188 opposite the base portion. A heat spreader 190, which in this example is implemented as a thermally conductive plate, is arranged to transfer heat to the heat dissipating portion of the enclosure.

An inductor is fabricated as part of a circuit board 192, which is shown as sections 192 a, 192 b, 192 c and 192 d because it includes slots to receive the legs of two magnetic core halves 194 and 196. The center legs of each core are shorter than the outer legs to provide an air gap 198 to prevent saturation of the core. The windings of the inductor are formed as conductive traces 200 which are formed on multiple layers of the circuit board and connected by vias which are not visible in this view.

The top core 194 may be coupled to the thermally conductive plate 190 through a thermally conductive pad 202. Alternatively, the top core 194 may directly contact the thermally conductive plate 190, it may be bonded with adhesive, or a thermal connection may be formed in any other suitable manner. The two magnetic core halves 194 and 196 may be held in place with adhesive, clips, or any other suitable technique.

The inventive principles illustrated with respect to FIG. 7 may be combined with the other inventive principles disclosed herein relating to removing heat from electric assemblies to create further synergistic results.

A potential advantage of the inventive principles relating to planar magnetics illustrated with respect to FIG. 7 is that they may enable the creation of reduced height structures that are cost effective and provide excellent manufacturing repeatability. Power magnetics cores in typical power electronics can reach elevated temperature. The inventive principles disclosed herein may provide reduced height structures while simultaneous enabling a magnetic core to be directly attached to a heat sink or through a thermal-pad. Alternatively a magnetic core may be attached to a heat sink through an electrically insulating material as described above.

FIG. 8 is a cross-sectional view illustrating an embodiment of an electrical assembly including through-board mounted components according to some inventive principles of this patent disclosure. The embodiment of FIG. 8 includes an enclosure 204 having a base portion 206 to attach the enclosure to a panel and a heat dissipating portion 208 opposite the base portion. A heat spreader 210, which in this example is implemented as a thermally conductive plate, is arranged to transfer heat to the heat dissipating portion of the enclosure.

A circuit board 212 is shown as sections 212 a and 212 b because it includes an opening for a through-board mounted component 214. The component 214 is mounted to the board through leads 216 which may be soldered to metal traces 218 a and 218 b on top of the board. Alternatively, or in addition, the component 214 may be mounted to the board through leads soldered to metal traces on the bottom of the board as shown with the broken lines in FIG. 8.

The through-board mounted component 214 may be thermally coupled to the thermally conductive plate 210 through a thermally conductive pad 202 as shown in FIG. 8. Alternatively, it may be thermally coupled through direct contact, or through any other suitable arrangement.

The inventive principles illustrated with respect to FIG. 8 may be combined with the other inventive principles disclosed herein to create further synergistic results.

A potential advantage of the inventive principles relating to through-board mounted components illustrated with respect to FIG. 8 is that they may further reduce the height of an enclosure in which the components are mounted.

The inventive principles of this patent disclosure have been described above with reference to some specific example embodiments, but these embodiments can be modified in arrangement and detail without departing from the inventive concepts. Such changes and modifications are considered to fall within the scope of the claims following the Appendices. 

The invention claimed is:
 1. An electrical assembly comprising: an enclosure having a base portion and a heat dissipating portion opposite the base portion; a circuit board having a first thermal interface on a first side of the board, a second thermal interface on a second side of the board, and a thermally conductive portion to provide enhanced thermal conduction between the first thermal interface and the second thermal interface; an electronic device, mounted on the first side of the board, having a thermal interface coupled to the first thermal interface of the circuit board; a heat spreader arranged to transfer heat to the heat dissipating portion of the enclosure; and a thermally conductive pad coupled between the second thermal interface of the circuit board and the heat spreader, wherein the heat spreader is positioned apart from the circuit board and positioned apart from the heat dissipating portion of the enclosure, wherein there is an absence of contact between the heat spreader and the heat dissipating portion of the enclosure, and wherein the enclosure does not comprise the heat spreader.
 2. The electrical assembly of claim 1 wherein the base portion has more than one sidewall, wherein the heat dissipating portion is a cover, wherein one or more of the sidewalls interfaces with the cover, and wherein the heat spreader does not directly contact the heat dissipating portion of the enclosure.
 3. The electrical assembly of claim 1 wherein the thermally conductive portion comprises a set of vias through the circuit board, at least some of the vias in the set of vias thermally connected to the first thermal interface and the second thermal interface.
 4. The electrical assembly of claim 1 wherein the thermally conductive portion comprises three vias through the circuit board, at least some of the vias thermally connected to the first thermal interface and the second thermal interface.
 5. The electrical assembly of claim 1 wherein the electronic device is a power inductor.
 6. The electrical assembly of claim 1 wherein the heat spreader is a thermally conductive plate, the thermally conductive plate positioned between the circuit board and the heat dissipating portion of the enclosure, the thermally conductive plate positioned apart from the heat dissipating portion of the enclosure.
 7. The electrical assembly of claim 1 wherein the circuit board is electrically connected, through the base portion, to a solar panel.
 8. The electrical assembly of claim 1, wherein the first side contains low-profile components and the second side contains high-profile components.
 9. The electrical assembly of claim 8 wherein the second side of the circuit board includes the electronic device in a plastic body and wherein the second thermal interface is a metallic slug.
 10. The electrical assembly of claim 1 further comprising: an electrically insulating sheet positioned between the circuit board and the heat spreader.
 11. An electrical assembly comprising: a box having a base portion to attach the enclosure to a panel and an opening opposite the base portion; a cover to engage the opening of the box; a circuit board having a first high-profile side with at least one power electronic device mounted thereon, and a second low-profile side; and a thermally conductive plate mounted adjacent to the second side of the circuit board; wherein the circuit board is constructed and arranged to transfer heat from the power electronic device to the thermally conductive plate; wherein the thermally conductive plate is arranged to transfer heat to the cover when the cover is engaged with the opening of the box and; wherein the thermally conductive plate is spaced apart from the circuit board and the cover.
 12. The electrical assembly of claim 11 wherein the panel is a solar panel.
 13. The electrical assembly of claim 11 wherein the construction and arrangement of the circuit board includes one or more vias positioned between a first thermally conductive pad and a second thermally conductive pad, the first pad and the second pad located on opposite sides of the circuit board.
 14. The electrical assembly of claim 11 wherein the circuit board is electrically connected to the panel through the box and wherein the panel is a solar panel.
 15. The electrical assembly of claim 11 wherein the circuit board is constructed and arranged to transfer heat from the first high profile side of the circuit board, through the circuit board, through a solder pad, through a thermally conductive pad and to the thermally conductive plate.
 16. The electrical assembly of claim 11 further comprising: an electrically insulating sheet positioned between the thermally conductive plate and the circuit board.
 17. The electrical assembly of claim 11 wherein the thermally conductive plate does not contact the cover.
 18. An electrical assembly comprising: an enclosure having a base portion to attach the enclosure to a panel and a heat dissipating portion opposite the base portion; a circuit board having a first thermal interface on a first side of the board, a second thermal interface on a second side of the board, and a thermally conductive portion to provide enhanced thermal conduction between the first thermal interface and the second thermal interface; a thermal interface, for transferring heat from an electrical component on the first side of the board, coupled to the first thermal interface of the circuit board; a heat spreader arranged to transfer heat to the heat dissipating portion of the enclosure; and a thermally conductive pad coupled between the second thermal interface of the circuit board and the heat spreader, wherein the heat spreader is a thermally conductive plate positioned apart from the heat dissipating portion of the enclosure and between the board and the heat dissipating portion of the enclosure, and wherein there is an absence of contact between the heat spreader and the heat dissipating portion of the enclosure, and wherein the enclosure does not comprise the heat spreader.
 19. The electrical assembly of claim 18 wherein the panel is a solar panel. 